Differences Between include and inherits in Puppet #

In Puppet, including a class (include) and inheriting a class (inherits) serve very different purposes.

Using include to Apply a Class #

Purpose and Usage #

• To apply a class to a node
• To ensure the class is declared only once, even if include is called multiple times
• To apply parameterized classes when used with class {} syntax

Key Characteristics #

• Executes the class immediately when it is included
• Works with parameterized classes when declared using class {} syntax
• Does not create a parent-child relationship

Example: Using include to Apply a Class #

class webserver {
  package { 'httpd': ensure => installed }
  service { 'httpd':
    ensure => running,
    enable => true,
    require => Package['httpd'],
  }
}

include webserver  # Applies the class

Example: Using class {} to Pass Parameters #

class webserver (
  String $package_name = 'httpd',
) {
  package { $package_name: ensure => installed }
}

class { 'webserver': package_name => 'nginx' }  # Applies the class with parameters

Using inherits to Create a Subclass #

Purpose and Usage #

• To create a child class that inherits properties from a parent class
• To extend an existing class by adding more resources or overriding variables
• Useful for defining default behaviors in a base class and extending them in child classes

Key Characteristics #

• Does not automatically apply the parent class
• Inherited class gets all resources from the parent but does not inherit parameters
• Cannot override parameters dynamically

Example: Using inherits to Extend a Class #

class base_webserver {
  package { 'httpd': ensure => installed }
  service { 'httpd': ensure => running, enable => true }
}

class custom_webserver inherits base_webserver {
  file { '/etc/httpd/conf.d/custom.conf':
    ensure => file,
    content => "Custom configuration",
  }
}

include custom_webserver  # This applies the child class

Comparison of include vs inherits #

Best Practices #

• Favor include over inherits in most cases
• inherits is rarely used in modern Puppet because it has limitations with parameterized classes
• Using Hiera with lookup() is usually a better approach for defining default behaviors instead of using inheritance